Perform some much-needed work on aescrypt that somehow lingered for
seven years...
- make aesget actually work with the default keysize (initialize it!)
- do not overflow the key filename buffers while parsing command-line options
- break out common code into a single file
- make the read()/write() loops a bit more robust
- fix lots of compiler warnings
- use snprintf() instead of strcpy()/strcat()
- write mdoc manual pages for aescrypt(1) and aesget(1)
- install the binaries and the manual pages with the correct permission mode